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permanent temporary DNS error in 50s11

A couple of days ago, a user here sent a piece of mail to the office.
At first he didn't get through. (The logs show a non-temporary "DNS
error" -- policystate -2.) Yet after 3 minutes later he got through (in
a resend) without any problems. That user said this problem had gone on
for a couple of days.

I was mystified by this. Digging through the source code, I did manage
to see (I hope) that policystate -2 is caused most likely by either an
-EX_NOHOST or a -EX_UNAVAILABLE from dnsmxlookup() (in mxverify.c), and
notice that there is the comment

          switch (hp->rcode) {
          case NXDOMAIN:
            /* Non-authoritative iff response from cache.
             * Old BINDs used to return non-auth NXDOMAINs
             * due to a bug; if that is the case by you,
             * change to return EX_TEMPFAIL iff hp->aa == 0.
            return -EX_NOHOST;

Since I don't understand the other two cases (and unfortunately I have
effectively turned off most logging in my bind), I guess I'll try to
patch this according to the comments and see if the problem goes
away. Alternatively, I wonder if anyone had noticed similar problems.
I believe my libresolv is 4.9.4 (from the Linux libc 5.4.46), but I
don't know if that's considered an old BIND. (The named is bind8, but
smtpserver is linked against libc, I believe -- is there a way to check?)

Ambrose C. Li / +1 416 321 0088 / Ming Pao Newspapers (Canada) Ltd.
EDP department / All views expressed here are my own; they may or
may not represent the views of my employer.